Mechansims of ion exchange across epithelia are being studied at levels of organization ranging from molecular to organismic. Emphasis is placed on the ontogeny of transport systems. Currently the differentiation of the Na ion and Cl minus transport systems in the skin of anurans is under intensive study. We hope to correlate the appearance of the transport system with specific biochemical and morphological changes in the skin. We are also engaged in a study of the mechanism of Cl transport across various epithelia including skin of adult anurans and urodeles and gills of larval amphibians.